The present invention extends the capabilities of near field acoustic microscopes by (1) using a new scanning probe design, (2) using much higher frequencies than previously used, thereby increasing lateral resolution and making accurate phase measurements possible, and (3) combining the capabilities of near field acoustic microscopy with those of atomic force microscopes, scanning tunneling microscopes, and/or other measurement methods.
Near field acoustic microscopy is one of several microscopy methodologies capable of atomic resolution microscopy. In near field acoustic microscopes the tip of a scanning prove is used to confine acoustic energy at high frequencies to a small diameter equal to that of the tip itself. The tip's diameter is much less than the acoustic wavelength. Acoustic energy quickly spreads out in the lateral direction as the spacing between the tip and a sample is increased. Therefore, in order to realize the full benefit of the confinement of acoustic energy by the narrow tip, it is necessary to observe the sample when it is placed very near the apex of the tip. Hence the name "near field" acoustic microscopy.
There has been previous work in this field. Zieniuk has transmitted sound waves through a narrow tip into water. The waves in the water propagate to the sample and enter the sample with a diameter that is more or less equal to the path length in water. In this situation the lateral resolution is determined by the spacing between the tip and sample. The system is quite inefficient since there is an impedance mismatch between the tip and water, and between the water and sample. J. K. Zieniuk, "Imaging Using Ultrasonic PIN Scanning Microscopy," IEEE Proceedings 1986 Ultrasonics Symposium, edited by B. R. McAvoy, Vol. 2, pp. 1037-1039. See also, W. Durr, D. A. Sinclair and E. A. Ash, "High resolution acoustic probe," Electronic Letters, Vol. 16, pp. 805-806, 1980.
Guthner and Dransfeld have demonstrated a second system by using the sharp corner of a tuning fork. With the tuning fork vibrating at its resonant frequency they brought it near to a sample and determined that the resonant frequency changed in the presence of the sample. The change was proportional to the spacing, and as they translated the sample beneath the tuning fork's corner the spacing would change, due to the topography of the sample's surface. Spacing changes were reflected by the changes in the resonant frequency. See (1) P. Guethner, E. Schreck, K. Dransfeld, "Scanning Tunneling Microscopy and Related Methods," edited by R. J. Behm et al., pp. 507-513 (chapter entitled "Scanning Nearfield Acoustic Microscopy"), Kluwer Academic Publishers, Netherlands 1990; and (2) P. Guthner, U. Ch. Fischer, and K. Dransfeld, "Scanning Nearfield Acoustic Microscopy," Applied Physics and Laser Chemistry, Applied Physics B., pp. 89-92, Springer-Verlag 1989.
Uozumi has demonstrated a system in which a bulk piezoelectric transducer was mounted in back of the tip of a scanning tunneling microscope (STM) and used this to excite sound waves at 1.4 MHz. These waves traveled down to the apex of the tip and returned. Thus, a reflection mode of operation was used. Uozumi found that the magnitude of the reflected signal was dependent on the spacing between the tip and sample. Kiyohiko Uozumi et al., "A Possible Novel Scanning Ultrasonic Tip Microscope," Japanese Journal of Applied Physics Vol. 28, No. Jul. 7, 1989 pp. L 1297-L 1299.
In still another experiment, Takata of Hitachi, Ltd., demonstrated a system where sound was transmitted from the tip through the sample to a piezoelectric detector place on the back side of the sample, thereby using a transmission mode of operation. The frequency used was a few kilohertz. See (1) Keiji Takata et al., "Tunneling acoustic microscope," Appl. Phys. Lett. 55(17), Oct. 23, 1989 pp. 1718-1720; and (2) Keiji Takata et al., "Electrostatic Force Imaging by Tunneling Acoustic Microscopy," Japanese Journal of Applied Physics Vol. 30, No. 2B, February 1991 pp. L 309-L 312.
The previously cited prior art methods for generating ultrasonic sound are cumbersome and inefficient. They are also restricted to low frequencies.
The present invention improves on the prior art acoustic microscopes by providing a scanning probe with a piezoelectric thin film transducer and using significantly higher ultrasonic frequencies, made possible by the use of a piezoelectric thin film instead of bulk piezoelectric transducers. It is desirable to operate a high frequencies, e.g., above 50 MHz, with short wavelengths. The inventors believe that the efficiency, and lateral resolution, of near field acoustic microscopy will improve as shorter and shorter wavelengths are used. Previous experiments have demonstrated that sound can be generated and detected at 100 GHz, with wavelengths in solids near 50 nanometers.
The present invention provides a number of different methods of exciting these high frequency sound waves, and detects the waves after interaction with a sample using a thin film of piezoelectric material such as Zinc oxide. These new modes of operation which expand the utility of acoustic microscopes, and combination AFM/acoustic microscopes and STM/acoustic microscopes.